This invention relates to valve operators and in particular to spring return valve operators which incorporate varying torque arm devices in the form of spools and flexible elements and also in the form of four bar linkage.
Spring return actuators are typically combined with controlled actuators such as pneumatic actuators, other fluid power actuators, electric actuators, or manual actuators to form failsafe valve operators. Failsafe valve operators protect against the failure of the control signal or the energy source for the controlled actuator and the subsequent possible hazards by returning the valve being controlled to a relatively safe operating position. The more well known hazards include fires, explosions and releases of toxic substances which have led to loss of life and property and to contamination of the environment, all of which is of increasing concern to industry, government, and the public. Spring return actuators are also combined with controlled actuators such as release and manual reset actuators to form valve operators for various safety type trip valves which are responsive to various process or environmental hazard conditions.
The prior art teaches making a pneumatic spring return valve operator by placing a spring power element consisting of one or more compression springs inside a pneumatic cylinder where it can act on the piston or by placing a compression spring outside the pneumatic cylinder where it can act on the piston rod. Such a combination is then used directly on sliding stem valves. For operating rotary valves such as quarter turn ball or butterfly valves, a linear to rotary motion converter mechanism is additionally provided in the valve operator. Well known linear to rotary motion converter mechanisms in the prior art include the slider crank, scotch yoke, rack and pinion, and various types of trunnion and clevis mounts.
Manually operated spring return actuators known in the prior art are generally limited to the smaller quarter turn valves and are based on compression springs or direct rotary motion springs.
There are a number of drawbacks to valve operators in the prior art due to interrelated problems with the character of the valve load, spring actuator output and controlled actuator output, with the efficiency with which the spring actuator is used, and with the size, weight and cost of the valve operator. These problems have caused spring return valve operators to be rather large heavy and expensive pieces of equipment.
Different kinds of valves present different types of loads to a valve operator. The valve load typically includes some factor of safety to assure reliable actuation of the valve. Some typical valve loads which are encountered include constant loads, high breakaway loads, loads which have high intermediate peaks, loads which decrease and then increase, etc. For example, quarter turn butterfly valves, depending on how the butterfly seats against the seals, can produce loads which increase, are constant, or are constant with a large increase at the end, as the valve moves toward the closed position. In addition, because of fluid pressure torqueing of the butterfly in some designs, the opening load can be different from the closing load. Quarter turn ball valves are generally taken as having a constant torque load for valve operator sizing purposes, although the load actually can dip somewhat in the middle of the actuation stroke and sometimes has a higher breakaway.
Within a valve operator in the prior art, a spring return actuator will produce a declining output as it actuates a valve. Typically, the extent of the decline is in the range of 20% to 50%. On the other hand, a pneumatic cylinder actuator will produce a constant force output. A vane actuator or a simple handle will produce a torque output which is approximately constant. Some of the linear to rotary motion converter mechanisms, such as the slider crank or the scotch yoke, will distort the output of pneumatic cylinder actuators and spring return actuators. For example, a pneumatic cylinder acting through a crank on a quarter turn valve will generate about 30% less torque at the beginning and end of actuation than in the middle. A rack and pinion type linear to rotary converter mechanism, on the other hand, generates a constant torqueing radius and thus the output torque bears a constant relation to the input force.
As a result, spring return actuators are not loadmatched to the valves that they operate and controlled actuators are not loadmatched to the spring return actuators that they have to retension and often are not loadmatched to the valve loads either. As the output of the spring return actuator must be greater than or equal to the valve load throughout actuation of the valve, this results in a spring return actuator which is, for the most part, oversized. Additionally, as the output of the controlled actuator must be greater than or equal to the maximum value of the valve load plus the retensioning load of the spring return actuator, this results in an oversized controlled actuator. The degree of valve load to actuator output mismatch represents excess actuator capacity. This excess capacity can have harmful side effects because high actuation accelerations and velocities in the valve can eventually damage the valve.
The degree of mismatch has been reduced in some prior art valve operators through the use of lower rate springs. There is, however, a direct tradeoff in the prior art between spring actuator size and weight versus degree of loadmatching. A lower rate spring which can meet a given valve load will be larger and heavier than a higher rate spring, but the higher rate spring will be less well loadmatched to the valve load and to the output of the controlled actuator than the lower rate spring thus forcing the use of a larger size controlled actuator. Additionally, the higher rate spring and the larger controlled actuator both increase the possibility of overstressing the valve. The situation in the prior art is further troubled by the fact that when a lower rate spring is used, much of the spring actuator capacity is used to maintain a preload. For example, I calculate that a spring actuator which is descirbed by Hooke's Law and undergoes a 20% change in deflection or decline in output during actuation, releases only 36% of of its energy. The use of high preloads not only represents excess capacity but also, in larger size valve operators, poses a safety problem to maintenance personnel having to release or apply such a preload.
I have attempted to summarize and illustrate in graph form the above problems and tradeoff in the prior art. FIG. 1 shows a plot of the output and load characteristics of a pneumatic spring return rack and pinion quarter turn valve operator and a constant torque load quarter turn valve. Line `VL` is the valve load. Line `S` is the spring actuator output and retensioning load as described by Hooke's Law. Line `P` is the pneumatic actuator output. Areas I, II and III taken together are a measure of the total energy storage capacity of the spring return actuator. Area I is the portion of the spring actuator energy capacity used to meet the valve load. Area II is the portion of the spring actuator energy capacity which is returned as preload and represents excess capacity because it is not used. Area III is the portion of the spring actuator energy capacity which is in excess of the valve load requirements and thus represents excess capacity. Areas I, III, IV and V taken together are the total amount of energy produced by the pneumatic actuator. Areas I and III are the portion of the pneumatic actuator energy capacity used to meet the spring actuator retensioning load but area III represents excess pneumatic actuator capacity because it restores excess spring actuator capacity. Area V is the portion of the pneumatic actuator energy capacity used to meet the valve load. Area IV is the portion of the pneumatic actuator energy capacity which is in excess of the valve load and spring actuator retensioning load requirements and thus represents excess capacity.
FIG. 2 shows the effect of using a lower rate spring, than that of FIG. 1, in the spring return actuator. While the maximum output of the spring return actuator has decreased and the output of the controlled actuator can in turn be decreased thereby decreasing the size of areas III and IV, the size of the spring and the preload energy, area II, have increased considerably.
The use of other linear to rotary converter mechanisms generally worsens the load mismatch and excess capacity problem in the prior art. FIG. 3 shows a pneumatic spring return actuator which acts through a slider crank. Line `Sc` is the torque output which would be generated by the spring actuator if the spring acted through a constant torque radius and through the same stroke. It shows the greater spring actuator capacity needed.
A valve, such as a butterfly valve, which presents an increasing load to a spring return actuator of the prior art will result in further load mismatch, as shown in FIG. 4.
The areas of excess actuator capacity are inherent and necessary in the prior art spring return valve operators in order to meet valve loads. In light of the invention, however, these areas of excess actuator capacity will be seen as unnecessary and wasted actuator capacity.
In addition the large size and weight of prior art spring return valve operators frequently results in considerable installation problems and costs. Often, pipes are of insufficient strength to support the large extra weight of a spring return valve operator and therefore extra supporting structure must be provided. Also, spring return valve operators frequently must be installed in confined places where the cost of providing extra space to accomodate the valve operator is high, as for example on a valve down in an underground vault or on a valve up in closely spaced pipe racks.